THE ELECTROWEAK THEORY CHRIS QUIGG Theoretical Physics Department Fermi National Accelerator Laboratory Batavia, IL 60510 USA E-mail: [email protected] After a short essay on the current state of particle physics, I review the antecedents of the modern picture of the weak and electromagnetic interactions and then un- dertake a brief survey of the SU(2)L U(1)Y electroweak theory. I review the features of electroweak phenomenology⊗ at tree level and beyond, present an in- troduction to the Higgs boson and the 1-TeV scale, and examine arguments for enlarging the electroweak theory. I conclude with a brief look at low-scale gravity. 1 Introduction 1.1 Our picture of matter At the turn of the third millennium, we base our understanding of physical phenomena on the identification of a few constituents that seem elementary at the current limits of resolution of about 10−18 m, and a few fundamental forces. The constituents are the pointlike quarks u c t , (1) d s b L L L and leptons ν ν ν e µ τ , (2) e− µ− τ − L L L with strong, weak, and electromagnetic interactions specified by SU(3) c ⊗ SU(2)L U(1)Y gauge symmetries. arXiv:hep-ph/0204104v1 9 Apr 2002 This⊗ concise statement of the standard model invites us to consider the agenda of particle physics today under four themes. Elementarity. Are the quarks and leptons structureless, or will we find that they are composite particles with internal structures that help us understand the properties of the individual quarks and leptons? Symmetry. One of the most powerful lessons of the modern synthesis of particle physics is that symmetries pre- scribe interactions. Our investigation of symmetry must address the question of which gauge symmetries exist (and, eventually, why). We must also under- TASI 2000 Lectures on Electroweak Theory: FERMILAB–CONF–01/001–T 1 stand how the electroweak symmetrya is hidden. The most urgent problem in particle physics is to complete our understanding of electroweak symme- try breaking by exploring the 1-TeV scale. Unity. We have the fascinating possibility of gauge coupling unification, the idea that all the interactions we encounter have a common origin—and thus a common strength—at suitably high energy. Next comes the imperative of anomaly freedom in the elec- troweak theory, which urges us to treat quarks and leptons together, not as completely independent species. Both ideas are embodied in unified theo- ries of the strong, weak, and electromagnetic interactions, which imply the existence of still other forces—to complete the grander gauge group of the unified theory—including interactions that change quarks into leptons. Su- persymmetry and the self-interacting quanta of non-Abelian theories both hint that the traditional distinction between force particles and constituents might give way to a unified understanding of all the particles. Identity. We do not understand the physics that sets quark masses and mixings. Although experiments are testing the idea that the phase in the quark-mixing matrix lies behind the observed CP violation, we do not know what determines that phase. The accumulating evidence for neutrino oscillations presents us with a new embodiment of these puzzles in the lepton sector. At bottom, the ques- tion of identity is very simple to state: What makes an electron an electron, a neutrino a neutrino, and a top quark a top quark? 1.2 QCD is part of the standard model The quark model of hadron structure and the parton model of hard-scattering processes have such pervasive influence on the way we conceptualize particle physics that quantum chromodynamics, the theory of strong interactions that underlies both, often fades into the background when the standard model is discussed. I want to begin these lectures on the electroweak theory with a clear statement that QCD is indeed part of the standard model, and with the belief that understanding QCD may be indispensable for deepening our understanding of the electroweak theory. Other lecturers will explore the application of QCD to flavor physics. Quantum chromodynamics is a remarkably simple, successful, and rich theory of the strong interactions.b The perturbative regime of QCD exists, aand, no doubt, others—including the symmetry that brings together the strong, weak, and electromagnetic interactions. bFor a passionate elaboration of this statement, see Frank Wilczek’s keynote address at PANIC ’99, Ref. 1. An authoritative portrait of QCD and its many applications appears in the monograph by Ellis, Stirling, and Webber, Ref. 2. TASI 2000 Lectures on Electroweak Theory: FERMILAB–CONF–01/001–T 2 1 10 100 1000 x=.0075 x=.0125 1.0 1.0 1.0 x=.0175 1.0 1.0 x=.025 1.0 1.0 x=.035 1.0 1.0 x=.050 1.0 1.0 x=.070 1.0 1.6 1.6 x=.090 1.4 x=.110 1.4 x=.140 1.2 1.2 x=.180 1.0 1.0 2 x=.225 F 0.8 0.8 x=.275 0.6 0.6 x=.350 0.4 0.4 x=.450 0.2 0.2 x=.550 x=.650 0.0 x=.750 0.0 1 10 100 1000 Q2 [GeV2/c2] 3 Figure 1. The structure function F2 measured in νN interactions, from Ref. thanks to the crucial property of asymptotic freedom, and describes many phenomena in quantitative detail. The strong-coupling regime controls hadron structure and gives us our best information about quark masses. The classic test of perturbative QCD is the prediction of subtle violations of Bjorken scaling in deeply inelastic lepton scattering. As an illustration of the current state of the comparison between theory and experiment, I show 2 in Figure 1 the singlet structure function F2(x, Q ) measured in νN charged- current interactions by the CCFR Collaboration at Fermilab. The solid lines for Q2 >(5 GeV/c)2 represent QCD fits; the dashed lines extrapolate to smaller values∼ of Q2. As we see in this example, modern data are so precise that one can search for small departures from the QCD expectation. TASI 2000 Lectures on Electroweak Theory: FERMILAB–CONF–01/001–T 3 Figure 2. Cross sections measured at √s = 1.8 TeV by the CDF Collaboration for cen- tral jets (defined by 0.1 < η1 < 0.7), with the second jet confined to specified intervals 4 | | in the pseudorapidity η2. The curves show next-to-leading-order QCD predictions based on the CTEQ4M (solid line), CTEQ4HJ (dashed line), and MRST (dotted line) parton distributions. Perturbative QCD also makes spectacularly successful predictions for hadronic processes. I show in Figure 2 that pQCD, evaluated at next-to- leading order using the program jetrad, accounts for the transverse-energy spectrum of central jets produced in the reaction pp¯ jet + jet + anything (3) → 1 2 over at least six orders of magnitude, at √s =1.8 TeV.c c 5 For a systematic review of high-ET jet production, see Blazey and Flaugher, Ref. TASI 2000 Lectures on Electroweak Theory: FERMILAB–CONF–01/001–T 4 0.20 CDF Preliminary 0.18 α s(MZ) = 0.1106 +/- 0.0001(exp.stat.) weighted average over E region 40 - 250 GeV 0.16 T 0.14 0.12 s α 0.10 α (E ) as function of E 0.08 s T T 0.06 0.04 µ MRSA´ parton distributions, =ET/2 Input α (M ) = 0.110 0.02 s Z 0 50 100 150 200 250 300 350 400 Transverse Energy [GeV] Figure 3. Determinations of αs inferred from the comparison of measured inclusive jet cross sections with the jetrad NLO Monte-Carlo program. Source of this figure is http://www-cdf.fnal.gov/physics/new/qcd/qcd99 pub blessed.html. The Q2-evolution of the strong coupling constant predicted by QCD, which in lowest order is 33 2n 1/α (Q2)=1/α (µ2)+ − f log(Q2/µ2) , (4) s s 12π where nf is the number of active quark flavors, has been observed within individual experiments6,7 and by comparing determinations made in different experiments at different scales.d Figure 3, from the CDF Collaboration, shows the values of αs(ET ) inferred from jet production cross sections in 1.8-TeV pp¯ collisions. The curve shows the expected running of the strong coupling constant. dFor a review, see Hinchliffe and Manohar, Ref. 8. TASI 2000 Lectures on Electroweak Theory: FERMILAB–CONF–01/001–T 5 12 LEP2 10 Ð Lattice e + e TRISTAN 8 PDG CLEO s α LEP 1/ 6 NMC UA2 GÐLS 4 PEP event shapes event Ð e + τ e 2 1 10 100 Q [GeV] Figure 4. Determinations of 1/αs, plotted at the scale µ at which the measurements were made. The line shows the expected evolution (4). Average Hadronic Jets e+e− rates e+e− event shapes Fragmentation Z width Small x structure functions ep event shapes Polarized DIS Deep Inelastic Scattering (DIS) τ decays _ QQ Lattice decay 0.090.10 0.11 0.12 0.13 0.14 0.15 α s(MZ) Figure 5. Determinations of αs(MZ ) from several processes. In most cases, the value measured at a scale µ has been evolved to µ = MZ . Error bars include the theoretical uncertainties. From the Review of Particle Physics, Ref. 9. TASI 2000 Lectures on Electroweak Theory: FERMILAB–CONF–01/001–T 6 A compilation of 1/αs determinations from many experiments, shown in Figure 4, exhibits the expected behavior.e Evolved to a common scale µ = MZ , the various determinations of αs lead to consistent values shown in Figure 5.